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Why do some cars keep on rolling when the engine is off and pressing the brakes

Tiny
DAVE213
  • MEMBER
  • 2007 BMW X6
  • 2.5L
  • 5 CYL
  • TURBO
  • 4WD
  • MANUAL
  • 2,639,300 MILES
Hi, I am a mechanic but not yet a certified one. I already been on this site before for my wife's car brakes. But this time I just want to know some thing. Why do some cars keep on rolling when the engine is off even when pressing the brakes?
Thanks for the answer.
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Thursday, March 12th, 2020 AT 1:51 PM

11 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,755 POSTS
I'm not sure I understand what you are asking. The brakes should work regardless. You will lose power assist, but they should still work. Do you mean why does the pedal get difficult to push? Also, can you provide a specific type of vehicle that you have noted this with?

Let me know.

Joe
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Thursday, March 12th, 2020 AT 4:45 PM
Tiny
DAVE213
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No I mean when the car is on park the brake pedal is easy to push and goes all the way down, but when start driving it goes back to normal again.
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Thursday, March 12th, 2020 AT 7:37 PM
Tiny
DAVE213
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I just want to know why I goes all the way down when in park.
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Thursday, March 12th, 2020 AT 7:37 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

It should never go all the way to the floor. That sounds like master cylinder issue. Since it feels normal when driving, it could be because you are pumping them. Take a look through this link.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Let me know.

Joe
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Thursday, March 12th, 2020 AT 7:55 PM
Tiny
DAVE213
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Wait I will send you a video. It's on my wife car before she starts to drive.
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Thursday, March 12th, 2020 AT 7:56 PM
Tiny
DAVE213
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That what I wanted to know it goes all the way down first, but she starts to drive it is normal again.
Video (Click to make bigger)
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Thursday, March 12th, 2020 AT 8:02 PM
Tiny
JACOBANDNICKOLAS
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You are most likely building pressure again in the system. At first, it may be bypassing the plungers in the master cylinder. After the first time, it pumps back up.

I definitely saw what you are talking about. If there are no leaks and the master cylinder is staying full, suspect it is bad.

Joe
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Friday, March 13th, 2020 AT 9:39 PM
Tiny
DAVE213
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Okay, then I will repair it. Thanks for the help.
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Friday, March 13th, 2020 AT 10:27 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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You are welcome. I wish I could be 100% sure, but that is what it is acting like. Here is a link that shows in general how one is replaced:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

I hope this helps you. Let me know if this takes care of the problem.

Joe
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Friday, March 13th, 2020 AT 10:49 PM
Tiny
DAVE213
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Thanks for the video.
Already on it.
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Friday, March 13th, 2020 AT 10:53 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Happy to help. Let me know if you need anything.

Take care,
Joe
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Saturday, March 14th, 2020 AT 6:00 PM

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